1. Field of the Invention
This invention relates to dithionites and more particularly relates to electrochemical preparation of dithionites from bisulfites.
2. Review of the Prior Art
Dithionites, commonly termed hydrosulfites, have been used for years to bleach a wide variety of materials including straw, feathers, glue, textiles, and wood pulps. For many such commercial uses in the past, zinc dithionite has been preferred because of its stability in aqueous solution, but ecological considerations in recent years have caused sodium dithionite to be used almost exclusively.
Chemical methods of manufacturing sodium dithionite are generally batchwise and require very careful supervision. An electrochemical process potentially offers many advantages. Particularly enticing are processes for the production of sodium dithionite solutions by cathodic reduction of sulfur dioxide. Reported anode and cathode redox reactions involve the reduction of aqueous sulfur dioxide solutions or sodium bisulfite solutions coupled with the oxidation of sodium chloride or sodium hydroxide solutions. However, the anode and cathode reaction systems therefor have limitations which cause poor conversion to product or product decomposition.
Typically, two-compartment cells are used for the electrolysis in which anode and cathode compartments are separated by a diaphragm or an ion-exchange membrane. In the cathode compartment of the electrolysis cell, aqueous sulfur dioxide is reduced to dithionite by the following half reaction: EQU 2SO.sub.2 +2e.sup.- .fwdarw.S.sub.2 O.sub.4.sup.2- (1)
In the anode compartment of the electrolysis cell, the electrolyte serves two functions. It is the source of sodium ions which are transported through a cation-exchange membrane to the cathode compartment, and it provides a source of easily oxidizable anions. Either a sodium chloride or a sodium hydroxide solution has been used to produce chlorine or oxygen, respectively, by one of the following half reactions: EQU 2NaCl.fwdarw.Cl.sub.2 +2Na.sup.+ +2e.sup.- (2) EQU 4NaOH.fwdarw.O.sub.2 +4Na.sup.+ +2H.sub.2 O+4e.sup.- (3)
Therefore, combining half-reaction 1 with either half-reaction 2 or 3 yields a sodium dithionite solution by either of the following overall reactions: EQU 2SO.sub.2 +2NaCl.fwdarw.Na.sub.2 S.sub.2 O.sub.4 +Cl.sub.2 (4) EQU 4SO.sub.2 +4NaOH.fwdarw.2Na.sub.2 S.sub.2 O.sub.4 +O.sub.2 +2H.sub.2 O (5)
When dithionite is produced according to reactions 4 or 5, the acidic catholyte environment of aqueous sulfur dioxide causes difficulties because the acid-catalyzed decomposition rate constant for the dithionite ion is large in the acidic conditions of the aqueous sulfur dioxide. This difficulty limits both product yield and current efficiency.
Alternatively, aqueous sodium bisulfite solutions can be used as the source of sulfur(IV). Bisulfite is reduced to dithionite at the cathode by the following half-reaction: EQU 2NaHSO.sub.3 +2e-.fwdarw.Na.sub.2 S.sub.2 O.sub.4 +2OH - (6)
By combining half-reaction 6 with either half-reaction 2 or 3, the following overall reactions result: EQU 2NaHSO.sub.3 +2NaCl.fwdarw.Na.sub.2 S.sub.2 O.sub.4 +2NaOH+Cl.sub.2 (7) EQU 4NaHSO.sub.3 +4NaOH.fwdarw.Na.sub.2 S.sub.2 O.sub.4 +4NaOH+2H.sub.2 O+O.sub.2 (8)
or EQU 4NaHSO.sub.3 .fwdarw.2Na.sub.2 S.sub.2 O.sub.4 +2H.sub.2 O+O.sub.2 (9)
Reaction 7 produces sodium dithionite and hydroxide at the cathode, while sodium chloride is consumed and chlorine gas is produced at the anode. Reaction 8 produces sodium dithionite and sodium hydroxide at the cathode, while sodium hydroxide is consumed and oxygen gas is produced at the anode.
Although both reactions 7 and 8 produce dithionite under pH conditions where dithionite is stable, the pH of the catholyte increases as the electrolysis proceeds. The pH of the catholyte will increase to the point where reduction becomes unfavorable, thereby limiting the bisulfite-to-dithionite conversion.
In practice, the addition of acid to the catholyte to neutralize the hydroxide produced at the cathode has not been found to be an acceptable solution to the problem. Injection of small volumes of dilute acid into the catholyte produced localized regions of low pH before mixing could produce a homogeneous solution. This condition resulted in significant losses of product due to acid-catalyzed decomposition.
In general, therefore, the traditional half-reaction combinations were found to limit the sulfur(IV) to dithionite conversion either due to dithionite decomposition from highly acidic electrolysis conditions or due to unfavorable thermodynamics as the catholyte pH increases.
U.S. Pat. No. 2,273,799 describes a process for producing sodium hydrosulfite from sodium bisulfite at a porous carbon cathode formed from comminuted solid carbonaceous material and a porous carbonaceous binder.
Two-chambered electrochemical cells, having cation permselective membranes which divide the cell into an anode compartment and a cathode compartment, have been described in U.S. Pat. No. 2,731,408 and may contain graphite and/or steel electrodes, as taught in U.S. Pat. No. 2,978,402.
Mechanical removal of the diffusion layer and electrolytic reduction of sodium dithionite, by use of the wiped-electrode technique with two-compartment cells, produced high yields of sodium dithionite, as reported in Chemical Communications, 1968, No. 7, pages 361-362. The anode compartment contained a graphite anode and saturated brine electrolyte. The cathode compartment contained a catholyte of NaHSO.sub.3, Na.sub.2 SO.sub.3, and NaCl and, selectively, a graphite cathode.
A process for making dithionites by electrolysis of an acidic solution of sulfur dioxide, utilizing permselective membranes separating anode and cathode compartments, has been described in Pulp and Paper Magazine of Canada in the issue of Dec. 19, 1969, at pages 73-78.
Experimental studies on the cathodic reduction of SO.sub.2 in aqueous solution are discussed in terms of the chemical and electrochemical reactions which accompany the flow of current in Journal of the Electrochemical Society, 1970, 117 (12), 1604-9.
U.S. Pat. No. 3,523,069 describes an electrolytic process for converting a solution of SO.sub.2 in water to an acidic solution of Na.sub.2 S.sub.2 O.sub.4 in the cathode compartment, the anode compartment containing an NaOH solution as anolyte and the cathode and anode compartments being separated by a cation-permeable membrane. Careful control of catholyte temperature at 0.degree.-40.degree. C., catholyte SO.sub.2 concentration at 2-25 wt. %, pH at 0.5-3.0, catholyte velocity at 1-40 meters/minute and other variables is required.
U.S. Pat. No. 3,748,238 describes a process for preparing sodium dithionite from sodium bisulfite or sodium metabisulfite in an electrolysis apparatus provided with a special spongy, porous lead electrode used therein as a cathode and substantially filling the catholyte compartment of the apparatus. The porous lead electrode is produced from alkali metal plumbites in the same electrolysis apparatus and remains in place for electrochemical preparation of sodium dithionite.
A process is described in U.S. Pat. No. 3,905,879 for making dithionites which begins with the production of high concentration, chloride-free sodium hydroxide solution and chlorine at a high current efficiency from a three-compartment electrolytic cell having cation-active permselective membranes separating anode and cathode compartments from a buffer compartment so that hydroxide ions migrate into the buffer compartment from a cathode compartment and are therein converted to sulfite by reaction with sulfur dioxide. The sulfite is removed and is subsequently added with additional sulfur dioxide to the cathode compartment of a two-compartment electrolytic cell wherein the cation-active permselective membrane separates the anode and cathode compartments. Chloride solution is electrolyzed to chlorine at the anode, and sulfite solution is electrolyzed to dithionite at the cathode.
U.S. Pat. No. 3,920,551 describes a process for making dithionites electrolytically by adding gaseous SO.sub.2 to the cathode compartment of an electrolytic cell in which the anode and cathode compartments are separated by a cation permselective membrane. The anode compartment contains an alkali metal chloride anolyte solution. In the cathode compartment, hydroxyl ions are reacted with SO.sub.4 to produce sulfite. The sulfite ions are then reduced to S.sub.2 O.sub.4.sup.32 at 3 to 5 volts, 0.1 to 2 A/in.sup.2, a catholyte pH of 6 to 8, an anolyte pH of 2-4, and a temperature of 5.degree.-20.degree. C.
A process for continuous manufacture of concentrated sodium dithionite solutions by cathodic direct reduction of solutions containing sulfite/bisulfite is also described in U.S. Pat. No. 4,144,146, within two-compartment cells divided by a chlorine-resistant cation exchanger membrane consisting of a copolymer of tetrafluoroethylene and a perfluorovinylsulfonic acid containing ether groups.
As can be seen from the preceding review, reported anode and cathode redox reaction systems have limitations which cause poor conversion to product or which cause product decomposition. These redox reaction systems for the electrolytic production of sodium dithionite solutions involve the reduction of aqueous sulfur dioxide solutions or sodium bisulfite solutions coupled with the oxidation of sodium chloride or sodium hydroxide solutions. There is clearly a need for a pH-stable electrochemical process.